1. Field of the Invention
The invention relates to an X-ray analysis apparatus, comprising an X-ray source, an X-ray detector and a specimen holder, and also comprising a goniometer which includes a system of rotary shafts for angular adjustment of the X-ray source and the X-ray detector relative to a specimen to be analysed.
2. Description of the Related Art
An X-ray analysis apparatus of this kind is known from U.S. Pat. No. 4,535,469 where it comprises a goniometer provided with a system of rotary shafts for rotation of a specimen relative to an incident X-ray beam (.theta.) and for coaxial rotation of a detector arm (2.theta.) for detection of a (diffracted) X-ray beam to be emitted by the specimen. To this end, such a goniometer comprises, for example a stepping motor and an angle transfer mechanism, or several stepping motors for mutually independent angular adjustment.
A goniometer must provide optimum effective detection of diffracted X-rays, for which exact knowledge of the angle is the variable determining the resolution of the apparatus. An X-ray analysis apparatus comprising such a mechanism has a complex and hence comparatively expensive construction and exhibits, inter alia because of the mechanical transmissions, a rather rigid angular adjustment and ratio of angles, notably also because it is necessary to reduce the lost motion in the rotations. This holds for apparatus comprising a specimen rotary shaft for .theta. adjustment in combination with a detection rotary shaft for 2.theta. adjustment, as well as for apparatus with a stationary specimen and rotary shafts for the X-ray source and the detector.
It is an object of the invention to provide an X-ray analysis apparatus in which exact but flexible angle detection and/or angular adjustment is possible and in which the occurrence of disturbing lost motion is avoided. To achieve this, an X-ray analysis apparatus of the kind set forth is characterized in that at least two shafts of the system of rotary shafts are provided with angle encoders for direct or indirect angle detection.
Because in an apparatus in accordance with the invention angle encoders are arranged between rotary shafts and, for example a frame of the goniometer, any desired angles can be exactly detected or adjusted, exact but flexibly adjustable coupling between angular motions is feasible, and angle detection can be simply performed on-line.
In a preferred embodiment, two rotary shafts are provided with an encoder disc, a read head which is mounted on the same stationary frame being associated with each encoder disc. As a result, the absolute value of the two angles is independently, directly determined and measurable and adjustable as such. The difference between the angles of the two shafts can be calculated from the two angular positions given. Such a construction is attractive, for example for X-ray diffraction apparatus comprising a specimen rotary shaft and a detector rotary shaft which are adjustable in the known .theta.-2.theta. geometry.
In a further preferred embodiment, a first shaft comprises an encoder which can be read relative to a stationary frame, a second rotary shafts being readable relative to the first rotary shaft. Using this construction, the angular position of the first rotary shaft as well as the difference between the angles of the two rotary shafts can be directly determined. The angular position of the second rotary shaft relative to a fixed position can be calculated, if desired, from said two values. Such a construction is attractive, for example for X-ray diffraction apparatus comprising a detector rotary shaft for adjustment of irradiation and detection angles relative to a stationary specimen. Preferably, the detector rotation is measured directly relative to a fixed position, because it is predominant for the accuracy of the measurement results.
In a further preferred embodiment, notably for X-ray diffraction apparatus, the encoders have defined measurement positions which can be accessed in accordance with the desired encoder accuracy during read out. In order to access a zero position, notably a microswitch is used for each of the encoders.
An encoder disc, which may be of an arbitrary type, is preferably arranged on a rotary disc mounted on a rotary shaft. Such an encoder disc should exactly indicate the shaft rotation and will, therefore, be selected in respect of exact working, finishing, coefficient of expansion and the like. Measurement through the disc is possible when an encoder disc is constructed so as to be optically transparent. For X-ray analysis apparatus, an encoder disc has a diameter of, for example 15 cm and is provided with, for example 720 lines, so one line per 0.5.degree.; using appropriate known read heads, measurement can be performed with an angular accuracy of up to 10.sup.-4 degrees. The measurement accuracy of the encoders, the construction thereof and the positioning of the read heads relative to a fixed point or to rotary shafts, and the definition of a zero position can be completely adapted to the relevant apparatus requirements. For example, for diffraction apparatus an exactly defined and exactly accessible zero position will be of greater importance to the measurement accuracy than for spectrometry apparatus where a standard specimen can be used.
A substantial advantage of the use of encoders in accordance with the invention is that the dynamic range of the angular motions can be simply adapted. For example, a slow approach and departure from an angular path can be simply realised, this preventing vibrations in the apparatus. Moreover, a desired sub-path, for example a peak searched in a spectrum, can be measured at any desired speed, and so forth.
The accuracy of the angular determination can be further enhanced by association of a plurality of read heads with the encoder disc, for example, two or three heads.